Biochemisty, Glutamate

Introduction

The function of glutamate as a neurotransmitter was long unknown because of the sheer abundance of the amino acid anions throughout the brain and nervous system and its complicated role in several metabolic pathways. It was studied for years regarding its use as a flavoring agent in monosodium glutamate, but it is now known to be the primary excitatory neurotransmitter in the human central nervous system, playing significant roles in learning, arousal, and behavior. Clinically, it is involved in addiction, psychosis, neurodegeneration and glial death. It has become a pharmacologic target in many areas of disease research.[1]

Fundamentals

Glutamate is the principal excitatory neurotransmitter of the central nervous system and the most abundant amino acid in the brain. It acts on a family of receptors and transporters (including NMDA, AMPA, kainite, and metabotropic receptors) found in the membranes of neurons and astrocytes to excite synapses and contribute to neuroplasticity. Excessive glutamate can cause excitotoxicity, and glutamate dysregulation is a possible feature in substance use disorders. The rate of its uptake from extracellular fluid largely regulates its concentration and actions in various compartments. Glutamate has clinical relevance in neurology and psychiatry, specifically regarding depression, substance use disorder, schizophrenia, neurodegenerative diseases, and other cognitive function and mood deficits.[1]

Issues of Concern

Of vital clinical importance are the implications of glutamatergic mediation in addiction and neurodegeneration. Chronic drug use can cause glial injury, which leads to glutamate dysregulation and eventually alters cortico-striatal transmission to incentivize drug-seeking behaviors. (PMID 16206168) Clinical evidence implicates glutamate’s excitotoxic effects in the pathophysiology of neurodegenerative and glial diseases, including Alzheimer’s disease and multiple sclerosis, which associate with decreased expression and altered localization of Na-dependent glutamate receptors in the central nervous system. Logistically, the ubiquitous nature of glutamate makes it difficult to quantify it in the body in a meaningful fashion. Despite its potential as a pharmacological target for both substance use disorders and neurodegenerative states, the glutamate pathway has historically posed problems to researchers, including serious adverse effects like increased stroke risk.[2][3]

Cellular

Glutamate connects the metabolic processes of neurons with those of astrocytes via the glutamate-glutamine cycle. Astrocytes, which account for a large proportion of brain mass and nutritional demand, act as a kind of compartment for up to 20% of the total glutamate in the brain. After neuronal release, glutamate accumulates in astrocytes, where it is stored as glutamine because of the activity of glutamine synthase, an enzyme not found in neurons. Astrocytes can also synthesize glutamate de novo and release it via exocytosis due to another enzyme not found in neurons, pyruvate carboxylase, which generates the glutamate precursor alpha-ketoglutarate. This synthesis seems to increase during brain activation, e.g., during memory formation. Neurons cannot synthesize glutamate or gamma-aminobutyric acid (GABA), the primary inhibitory neurotransmitter, de novo from glucose. Instead, they hydrolyze glutamate to glutamine via mitochondrial glutaminase at glutamatergic synapses or convert it via glutamate decarboxylase to GABA.[4]

Molecular

The glutamate/GABA-glutamine cycle is a bi-directional transfer of carbon and nitrogen units, by which neurons convert glutamine precursor into glutamate and GABA neurotransmitters. Astrocytes, in turn, regenerate glutamate using glutamine and free ammonia, possibly released from neurons following their deamidation of glutamate at synapses, or synthesize glutamate de novo from glucose. The amount of de novo synthesis depends largely on the net loss of glutamate via oxidative reactions contributing to pyruvate recycling. Glutamate is metabolized through the tricarboxylic acid cycle in both neurons and astrocytes, albeit to different extents. Five high-affinity glutamate transporters are known: EAAT1 (GLAST), EAAT2 (GLT-1), EAAT3 (EAAC1), EAAT4, and EAAT5. EAAT1 and 2 are primarily responsible for glutamate uptake in the brain, coupled with three Na and one H entering the cell and one K leaving, which maintains a 1,000,000:1 inside-out concentration gradient.[5]

Function

Glutamate has all the hallmarks of a conventional neurotransmitter, plus the added complexity of the neuronal-astrocytic interplay that contributes to its recycling and to regulating excitatory brain activity. The excitatory neurotransmitter contributes to the consolidation of memories, which is the basis of long-term learning, via activation of NMDA, AMPA, and metabotropic receptors in the brain. Because of AMPA and metabotropic receptors on astrocytes, glutamate can reinforce synaptic neuronal activity (and, presumably, learning via synaptic plasticity) and recruit resting neurons into activity. Glutamate has been implicated in kainate-mediated excitotoxicity due to increased cytosolic free Ca concentration, release of mitochondrial cytochrome c, and apoptotic cell death.[6]

Mechanism

Glutamate, a free amino acid and the most abundant excitatory neurotransmitter in the human central nervous system, is the nexus between multiple metabolic pathways. It is concentrated in the synaptic vesicles of neuronal terminals, from which it is released by exocytosis, anion channels, and transporter reversal. It cannot cross the blood-brain barrier and is continuously removed from the extracellular fluid by astrocytes to prevent excessive receptor activation. Glutamate acts on several types of receptors, including excitatory amino acid transporters (EAATs 1-5), vesicular glutamate transporters (VGLUTs), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors, N-Methyl-D-aspartate (NMDA) receptors, glutamine-cystine exchangers (xCT), and various intracellular carriers; one or more of these receptors are expressed on nearly every cell in the nervous system. EAATs mediate excitatory signaling, including toxicity, while AMPA and NMDA receptors have been identified as mediators of synaptic plasticity and thus learning and learned behavior. NMDA receptors are especially important, playing an instrumental role in directing cortical migration. Glutamate excitation is tightly regulated, in large part via the astrocytic/neuronal glutamate-glutamine or glutamate-GABA/glutamine cycle.[1][7]

Testing

The primary methods of determining glutamate and glutamatergic receptor quantity and distribution in the body are tissue culture & fMRI. These techniques orient more toward research rather than clinical use, due to their high expense and invasiveness. Logistically, the ubiquitous nature of glutamate in the body makes it difficult to test quantitatively.[4]

Pathophysiology

Glutamate is of importance in several pathophysiological disease processes. Excitotoxicity mediated by increased cytosolic Ca, cytochrome c release, and apoptosis is both acutely traumatic and chronically contributive to neurodegenerative diseases, probably through both cell death and pro-inflammatory pathology. Glial injury also contributes to these pathways. Diseases in question include Alzheimer’s, multiple sclerosis, amyotrophic lateral sclerosis, and more.[6][8] The pathophysiology of addiction also involves glutamate, whose homeostatic cycling becomes dysregulated with chronic drug use. Eventually, this leads to a breakdown of communication between the pre-frontal cortex and the nucleus accumbens and reinforces patterns of drug-seeking.[9] Dysfunction of NMDA receptors and resultant glutamate deregulation has implications in the pathophysiology of schizophrenia, but the body of research regarding this topic is less robust. However, NMDA receptor antagonists like memantine and clozapine are important pharmacological targets in the treatment of schizophrenic patients.[10]

Clinical Significance

Excitotoxicity, for which glutamate is considered the culprit, is involved in diabetic retinopathy and other health conditions. Glutamate’s role in synaptic plasticity has broad clinical implications, particularly around learning and functional disabilities, neurodegenerative diseases, and addictive behaviors.[6] As a pathogenetic factor in substance use disorder, dysregulation in glutamate homeostasis has been shown to disrupt communication between the pre-frontal cortex and the nucleus accumbens, changing the capacity for neuroplasticity and altering dendritic spine morphology. Repeated drug use leads to an imbalance between synaptic and non-synaptic (glial) glutamate, which impairs corticostriatal control of drug-seeking behavior. This pathway also raises potential pharmacological targets; already, N-acetylcysteine has been shown to inhibit drug-seeking relapse.[9] Excitotoxicity-induced glial injury can also mediate neurodegeneration by altering expression of Na-dependent glutamate transporters, leaving the brain more susceptible to aberrant glutamate cycling, and contributing to diseases affecting both neurons (e.g., dementia) and glia (e.g., multiple sclerosis) alike.[2]

Glutamate contributes to neuronal damage via increased free Ca, the release of mitochondrial cytochrome c, and apoptotic cell death.[6] It affects each type of glial cell differently. Oligodendrocytes, which are very sensitive to excess glutamate, undergo apoptosis via an AMPA receptor-mediated process similar to that of neurons. Astrocytic injury potentiates glutamate dysregulation, and microglial injury can further impair neuronal glutamate uptake and receptor expression. Besides substance use disorder and neurodegeneration, these pathways have implications for traumatic brain injuries and brain ischemia.[8] For these reasons, glutamate receptor expression and function, including that of GLT-1, has been identified as a potential pharmacological intervention target.[11] Outside the nervous system, glutamate may play a role in osmotic signaling that controls whole-body protein metabolism, which has important implications for skeletal muscle maintenance and cardiac function.[12]